It is known, in the measurement of travel time differences of ultrasonic pulses, to utilize ultrasonic transducers as transmitter and also as receiver in flow field investigations in wind tunnels. The ultrasonic transducers have a relatively large housing and require extensive mountings and special current supplies. The considerable spatial extent of such an ultrasonic transducer and the associated auxiliary devices often leads to disturbances in the wind tunnel which make the measurement results useless. Particularly in closed tunnels, a great change in the flow cross section results from the arrangement of the measuring equipment, such that in many cases the measurement results are unreasonably distorted. Also, several successive measurements often have to be made to survey a flow field, and the ultrasonic transducer has to be displaced between them. Only stationary flows can be investigated in this manner, or such instantaneous processes in which knowledge of the expected course of the flow process with time (e.g., in the special case of periodicity) can solve the measuring process at the respective correct instant of time. Here, however, relatively long measurement times are concerned. The possibilities of use of such an ultrasonic transducer are strongly restricted to special cases. Since the ultrasonic transducer must couple the ultrasonic pulses to the measurement region, the flow field is impaired both at the beginning, i.e. at the input coupling point, and also at the end, by the presence of the ultrasonic transducer. This impairment only disappears when a smaller distance can be taken as the integration region than the distance between the two ultrasonic transducers, which can be achieved in practice in special cases by corresponding measures.
An ultrasonic transducer is disclosed in Federal Republic of Germany Patent No. DE-PS 31 03 357. in which special receiving microphones are used which must vibrate in resonance with the incoming signal. The change of the integration path in the flow field requires a considerable cost in apparatus and modification of the measurement device. In each case there is required in the flow field a receiving microphone as a probe, which in the special case can also be arranged outside the measurement region. A temperature measurement is not possible without insertion of a probe into the measurement region or at the measurement point.
Known laser measurement processes (e.g., laser doppler, two-focus procedure, etc.) do not permit any integration path to be built up or any instantaneous measurements to be carried out. The time required to survey a flow field with the usual laser measurement processes is considerably greater than with the ultrasonic measurement method.
The object of the invention is to teach a process and an apparatus by means of which travel time differences can be measured, in the measurement region of interest without contact (without disturbing probes), for ultrasonic pulses, and the measurement region can be selected without influencing the flow.
Stationary and instantaneous flow fields are to be measured such that path integrals of flow components can be determined, e.g. for the estimation of vortex parameters. On the one hand it is to be possible to integrate over long paths (e.g. in detection of vortices), and on the other hand it is also to be possible to integrate over short paths, in order to achieve a more or less pointwise determination of flow components. Since the temperature is important from various viewpoints in flow processes, in particular with compressive flow, it is also to be possible to determine this temperature. The equipment required for the process is also to be able to be fitted, without disturbance, outside the flow field to be investigated, and in fact also in closed wind tunnels. Furthermore, it is to be possible to make measurements at, or from, great distances. By a rapid sequential or even parallel operation of the equipment, instantaneous as well as stationary flow processes are to be recordable.